Selective area solder placement

ABSTRACT

A printed circuit board having at least one layer of conductive traces on an external surface has at least one preformed solder element placed on a conductive trace area of the printed circuit board requiring a greater than standard amount of solder. The at least one preformed solder element is reflowed to form a connection with the layer of printed solder.

RELATED APPLICATION(S)

This application is a Divisional of U.S. application Ser. No. 10/192,346filed Jul. 10, 2002, which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to electronic printed circuit boards,and more specifically to placing solder deposits on the printed circuitboard.

BACKGROUND OF THE INVENTION

Electronic components are typically assembled into complex circuits bymounting them on printed circuit boards. These printed circuit boardsare usually flat nonconductive boards with one or more layers of aconductive material such as copper fixed on or in the printed circuitboard. The layers of copper are etched or otherwise formed to specificshapes and patterns in the manufacturing process, such that theremaining conductive copper traces are routed to connect electricalcomponents to be attached to the printed circuit board.

It is not uncommon for printed circuit boards such as these to havecopper layers on both a top and bottom side of the circuit board, butalso several layers of copper traces or patterns sandwiched at variousdepths within the circuit board itself. These traces allow greaterflexibility in circuit routing, and usually allow designing a morecompact circuit board for a particular circuit than would otherwise bepossible. The various layers are sometimes dedicated to particularpurposes, such as a ground layer that only serves to distribute groundor signal return connections to various components.

Circuit board layers that serve to connect components to othercomponents often must connect to components attached to a differentlayer (top or bottom), or such layers must be attached to each other atselected points, requiring use of what are commonly known as vias. Thesevias typically are essentially small conductive plated-through holeelements oriented perpendicular to the top and bottom surfaces of acircuit board that extend through at least two conductive layers of thecircuit board, and that electrically connect circuit traces on at leasttwo of the conductive layers to each other. These vias may be filledwith solder during the solder printing process of printed circuit boardfabrication, if a method is used that enables application of sufficientsolder to fill the hole.

Connection of components such as through-hole mounted (THM) componentsor solder-connected shields to the printed circuit board can alsorequire a relatively large amount of solder to ensure a reliableconnection, and therefore ideally will use more solder than a typicalprinted circuit board printing process will provide. Some ceramiccomponents such as ball-grid arrays (BGAs) or other surface mountdevices may also benefit from more solder than is typically ideal formost of the rest of a printed circuit board, further evidencing a needfor application of a relatively large amount of solder to selected areasof a circuit board.

Solutions to this need for additional solder on some certain areas ofprinted circuit boards typically involve increasing the density of thesolder applied to all areas of the board in the solder printing process,or use of various types of stencil to apply additional solder to theboard. But, because of the fine pitch of interconnects and componentconnection pads on many printed circuit boards, application of arelatively large amount of solder throughout a circuit board is notdesirable because it tends to cause solder bridging between traces andpads on the circuit board. Similarly, use of thick stencils requires aspacing of typically 4 mm around the area to which solder is to beapplied, can impact other surface mount components on the circuit board,and is rather limited in the amount of solder than can be applied.Overprint stencils are further limited in the solder volume that can beapplied, and so are also undesirable for many applications.

All current known solutions to the need for additional solder on certainareas of a printed circuit board involve the printing process, and arelimited by the technology of the solder printing apparatus which was notdesigned to vary the amount of solder printed across a circuit board.Use of stencils and extra-thick solder application are not appropriatesolutions for applications involving fine pitch circuit traces and pads,or for many surface mount technology applications.

What is needed is a technology enabling selective application of anamount of solder greater than is normally applied in a solder printingprocess to selective areas of a printed circuit board.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a stepped stencil for use in applying solder to a printedcircuit board, consistent with the prior art.

FIG. 2 shows an overprint stencil for use in applying solder to aprinted circuit board, consistent with the prior art.

FIG. 3 shows a printed circuit board printed with a thin layer ofsolder, consistent with an embodiment of the present invention.

FIG. 4 shows a printed circuit board with a printed layer of thin solderand a preformed solder element placed on the printed layer, consistentwith an embodiment of the present invention.

FIG. 5 shows an electronic component placed on a printed circuit boardwith preformed solder elements, consistent with an embodiment of thepresent invention.

FIG. 6 shows a printed circuit board with a shield attached thereto andhaving preformed solder elements placed thereon, consistent with anembodiment of the present invention.

FIG. 7 shows a computer system, consistent with an embodiment of thepresent invention.

DETAILED DESCRIPTION

In the following detailed description of sample embodiments of theinvention, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificsample embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that logical, mechanical,electrical, and other changes may be made without departing from thespirit or scope of the present invention. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the invention is defined only by the appended claims.

The present invention provides in various embodiments a technologyenabling selective application of an amount of solder greater than isnormally applied in a solder printing process to selective areas of aprinted circuit board by placement of preformed solder elements to theprinted solder layer on a printed circuit board. The preformed solderelement is reflowed, connecting the printed circuit board's conductivetraces to electrical components.

FIG. 1 illustrates a prior art method of applying large amounts ofsolder to a printed circuit board. The stencil 101 has openings 102 and103 through which solder is applied. The solder flows into the cavitiesformed by the holes 102 and 103 in the stencil, and is restricted fromflowing out the bottom of the stencil by contact between the stencil anda circuit board. Because the stencil is thicker at 103 than at 102, agreater amount of solder will be applied to the underlying circuit boardstructure at 103 that at 102. The ability to vary the amount of solderapplied to circuit board features in this manner is limited toapproximately a 0.004 inch change in stencil thickness, as pictured inthe ‘Y’ dimension shown at 104. Further, the step in the stencil willcause a greater amount of solder than is applied at 102 to be applied inother apertures within about 0.04 inches of the change in stencilthickness, shown in the ‘X’ direction at 105. These limitations not onlyrestrict the amount of solder that can be applied with this method, butimpose design restrictions on the rate at which solder thickness can bechanged across a circuit board.

FIG. 2 shows an overprint stencil 201 for use in applying solder to aprinted circuit board. Solder is simply printed through stencil opening202, which is greater in width than the underlying circuit boardfeature, which is therefore overprinted with solder. The amount ofoverprinting that can be practically achieved with a stencil such as 201is limited by the amount of solder pullback that can be reliablyexpected after solder printing, which limits overprinting to withinabout 7 mm of a printed circuit board feature needing a greater thanstandard amount of solder. The amount of solder applied is also limitedin some instances by the relative narrow size of the opening 202 andlimited practical thickness of the stencil, or by the aperture ofopenings 202.

FIG. 3 illustrates printing a first layer of solder to a printed circuitboard 301 with a conventional printing process. The thickness of thesolder 302 that is applied is limited by the printing equipment such asthe stencil thickness, and is practically limited by the pitch of thenarrow, dense circuit board traces and pads. The solder 302 illustratedhere is applied to a pad surrounding through-hole 303 in the printedcircuit board.

FIG. 4 shows how one embodiment of the present invention's use ofpreformed solder elements allows application of a greater amount ofsolder than was possible with other methods. Printed circuit board 401has though holes such as 402 covered with solder paste 403. The solderpaste acts as a sticky adhesive to which preformed solder element 404 isplaced such as by using standard electronic component placementequipment. This placement equipment may include a standard chip shooter,a fine pitch pick and place machine, a standard or specially designedvacuum nozzle, or any other method of placing a component on a circuitboard. The solder element 404 is here a washer-shaped solder elementwith a hole in the middle, placed so that the hole in the middle of thesolder element is aligned with the through hole 402 of the printedcircuit board. The solder element may take any shape, including solidshapes such as rectangles, cylinders, or spheres, or other shapes thatare not solid such as washers, toroids, and rectangles with holes.

FIG. 5 shows how an electrical component such as a connector 501 canthen be applied to a circuit board with preformed solder elements suchas was shown in FIG. 4. Here, the connector 501 is placed on the printedcircuit board 502 such that the pins of the connector 503 extend throughthe through hole 402 and the hole in the washer-shaped preformed solderelement 504. The preformed solder element 504 is then heated or reflowedso that the solder melts, forming an electrical connection between theconnector component 501 and conductive traces on the printed circuitboard. The preformed solder element 504 therefore serves to provide agreater amount of solder to the area of connection between connector 501and the conductive trace on circuit board 502 than is standard, such asby typical solder printing methods.

FIG. 6 illustrates how such solder elements can be placed in anotherapplication to support a shield element on a printed circuit board.Here, a printed circuit board 601 shown from the top has upon it severalcopper traces 602 electrically connected to a ground potential. Apreformed solder element 603 is placed on each copper trace adjacent toits point of connection to the attached shield 604. The solder is heatedor flowed to cause it to both physically and electrically connect theshield 604 to the copper traces 602 on the circuit board 601.

FIG. 7 illustrates a computerized system, consistent with an embodimentof the invention. A motherboard 701 has various components attachedthereto, including a processor 702, memory 703, and expansion slots 704.The motherboard of one embodiment of the invention comprises at leastone preformed solder element used to supply a greater than standardamount of solder for attachment of one or more components such assockets for memory 703 or expansion slots 704 to the motherboard. Inanother embodiment of the invention, one or more solder elements areused to attach a shield, such as shield 705, to the motherboard.

It has been shown in these examples how placement of a preformed solderelement, such as with an electrical component pick-and-place machine,easily and efficiently provides additional solder to regions of aprinted circuit board such as physically large connection points thatrequire more solder than is applied in a typical solder printing system.The embodiments of the inventive method described herein allows agreater amount of solder to be placed than do other traditional methodssuch as overprinting, and can be implemented largely using existingequipment.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of the invention. It isintended that this invention be limited only by the claims, and the fullscope of equivalents thereof.

1. A method of producing a printed circuit board, comprising applying alayer of printed solder to conductive trace areas of the printed circuitboard requiring a standard amount of solder; placing at least onepreformed solder element on at least one conductive trace area of theprinted circuit board requiring a greater than standard amount ofsolder; and reflowing the at least one preformed solder element to forma connection between the layer of printed solder and the at least onepreformed solder element.
 2. The method of claim 1, wherein the at leastone preformed solder element has a hole passing through the solderelement.
 3. The method of claim 1, wherein the at least one preformedsolder element is a rectangular solder element.
 4. The method of claim1, wherein the placing at least one preformed solder element on aconductive trace area of the printed circuit board is performed usingelectronic component placement equipment.
 5. The method of claim 1,wherein the reflowing the at least one preformed solder element occursafter placement of components on the printed circuit board.
 6. Themethod of claim 5, wherein the reflowing the at least one preformedsolder element forms a connection between the printed layer of solder,the at least one solder element, and a component placed on the printedcircuit board.
 7. A method of applying solder to a printed circuitboard, comprising: placing at least one preformed solder element on atleast one conductive trace area of the printed circuit board; andreflowing the at least one preformed solder element to form a connectionbetween the layer of printed solder and the at least one preformedsolder element.
 8. The method of claim 7, wherein the at least onepreformed solder element has a hole passing through the solder element.9. The method of claim 7, wherein the at least one preformed solderelement is a rectangular solder element.
 10. The method of claim 7,wherein the placing at least one preformed solder element on aconductive trace area of the printed circuit board is performed usingelectronic component placement equipment.
 11. The method of claim 7,wherein the reflowing the at least one preformed solder element occursafter placement of components on the printed circuit board.
 12. Themethod of claim 11, wherein the reflowing the at least one preformedsolder element forms a connection between the printed layer of solder,the at least one solder element, and a component placed on the printedcircuit board.